Electrical transients on known power circuits are a major cause of problems especially in sensitive electronic equipment. The higher the voltage level of an electrical transient which reaches equipment to be protected, the more likely the equipment is to be damaged or adversely affected by the transient; for example, on 240 volt A.C. mains supply, transient over voltages can be caused by switching, remote lightning strikes and the like. The mains supply may carry transient over voltages of as much as 10 kilovolts, for example, and it is necessary to ensure that the over voltage which reaches the load equipment is reduced substantially, preferably below 1000 volts, suitably below 700 volts.
Various proposals have been made to protect installations against transient over voltages but these have often been expensive and/or not altogether effective: for example in some proposed protective apparatus, a leakage of a small current from the live conductor to earth may occur, which is undersirable. A voltage dependent resistor, known as a varistor, rated for operation on 240 volt mains will leak a steady-state current of less than 1 mA. However whilst diverting a 1 kA transient the varistor will develop in excess of 900 volts across its terminals. A varistor (or combination rated in total for 190 volt A.C. will develop only about 620 volts for the same transient.
Although the 190 volt rated component clearly offers better protection, it cannot be used on 240 volt mains since the steady-state leakage current would lead to over-heating and destruction of the varistor.
It has been proposed to protect electrical equipment against electrical transients by devices including gas discharge tubes and voltage dependent resistors. For example GB2175156A describes one such apparatus but in this apparatus voltage dependent resistors are connected directly between the live and neutral supply leads and, therefore, will suffer from current leakage as outlined above, or provide poor protection against live/neutral transients. Furthermore, the protection against transients occurring between live and neutral conductors is relatively ineffective, relying on the voltage dependent resistors. U.S. Pat. No. 4,677,518 describes another such apparatus but this is designed to give protection between only two conductors (live and neutral) and no direct protection between live and earth nor between neutral and earth conductors is envisaged.